The subject invention relates to microwave probes useful in hyperthermia therapy, and more particularly, to probes utilizing a coaxial antenna construction and useful primarily for the interstitial or invasive hyperthermia treatment of tumors.
It has long been known that certain cancer cells can be destroyed at elevated temperatures which are slightly lower than temperatures normally injurious to healthy cells. In recent years, hyperthermia therapy utilizing electromagnetic radiation has been found to be particularly effective and many processes and types of apparatus utilizing microwave hyperthermia are known in the art. These include non-invasive types in which external applicators are utilized and the microwave energy is allowed to penetrate the skin and underlying tissue, including a tumor to be treated. Obviously, this results in the heating of healthy tissue as well and this lack of control is one reason why more direct and exact means of applying microwave hyperthermia have been sought.
Thus, invasive methods and related apparatus have been developed in which a hyperthermia probe or probes may be inserted to the point of treatment via a normal body opening or may be inserted interstitially through the skin directly to the site of the tumor to be treated. Such invasive methods and apparatus provide the advantage of potentially better control of temperature of the mass or volume of tissue to be treated.
Since many types of malignancies cannot be reached and effectively treated with non-invasive techniques or with invasive probes designed to be inserted into a normal body opening, much attention has been recently given to long, narrow needle-like probes which can be inserted directly into the body tissue and to the site of the tumor or malignancy to be treated. Such probes must of necessity be of a very small diameter, both to aid the ease with which they may be inserted and used and to reduce the trauma associated therewith to the patient.
Interstitial microwave hyperthermia probes may be of a rigid or semi-rigid type with a needle-like point for direct insertion into the body tissue. Alternately, the probe may be more flexible and adapted to be used inside a catheter first inserted into the body tissue by ordinary, well-known methods. The advantages of using a probe inserted into a catheter include avoiding the need to separately sterilize the probe and to take advantage of catheters which may already have been inserted for other types of concurrent treatment, such as radiation therapy. Nevertheless, the convenience of more rigid probes adapted for direct interstitial insertion allow their alternative use in certain situations.
Regardless of the type of microwave probe, a problem common to all of them has been to provide a uniform pattern of radiated energy and heating axially along and radially around the effective length of the probe or to otherwise control and direct the heating pattern. A known and predictable heating pattern is, of course, important so that the heating may be confined to the greatest extent possible to the tissue to be treated and excessive heating of healthy tissue avoided.
Microwave probes of two kinds have been used, one comprising a monopole microwave antenna and the other a dipole coaxial antenna. In a monopole antenna probe, a single elongated microwave conductor is exposed at the end of the probe (sometimes surrounded by a dielectric sleeve) and the microwave energy radiates generally perpendicularly from the axis of the conductor. However, so-called monopole probes have been found to produce non-uniform and often unpredictable heating patterns and the heating pattern does not extend beyond the probe tip. As a result, more recent attention has been directed to so-called "dipole" antennas of a coaxial construction. These include constructions having an external reentrant coaxial "skirt" around the distal end of the outer conductor.
The typical coaxial probe includes a long, thin inner conductor extending along the axis of the probe, surrounded by a dielectric material, and an outer conductor surrounding the dielectric. To provide the effective outward radiation of energy or heating, a portion or portions of the outer conductor can be selectively removed. This type of construction is sometimes referred to as a "leaky waveguide" or "leaky coaxial" antenna. Obviously, variations in the location, size and area of the outer conductive material removed along the effective length of the probe can significantly affect the heating pattern provided. One of the primary goals in such construction has, thus, been to provide a uniform heating pattern generally or a more narrowly controlled and directed pattern in a selected region of the probe tip. U.S. Pat. No. 4,204,549 discloses the removal of short semi-cylindrical sections of the outer conductor in a coaxial probe to provide directional control of the heating pattern, but uniformity in the control of the heating pattern is not discussed. U.S. Pat. No. 4,669,475 discloses a coaxial antenna probe in which a full circumferential cylindrical portion of the outer conductor is removed over a selected intermediate axial length of the probe. However, the heating pattern per se is not disclosed or described, and the heating pattern of individual or multiply oriented probes is controlled by varying the microwave energy supplied to the probes. U.S. Pat. No. 4,658,836 discloses the removal of long axial segments of the outer conductor to provide full length heating. Control of the heating pattern is attained by varying the thickness of supplemental outer dielectric covering or by a unidirectional variation in the axial outer conductor segments. In an alternate embodiment, the outer conductor is attached in a uniform double spiral pattern to provide the necessary open space for radiation leakage. However, the utility of the spiral pattern is disclosed as providing the probe with flexibility and to provide relative rotation of the heating pattern along the probe length. Finally, the probe of this patent is intended particularly for insertion into a body passage or cavity and not direct interstitial insertion into body tissue.
Control of the heating pattern in coaxial probes for interstitial hyperthermia treatment continues to be a problem. It is important to be able to control and predict the heating pattern axially along the effective length of the probe. Because of the need to maintain the very small diameter of these probes, it is impractical to utilize heating pattern control means which increase the effective diameter, such as an outer dielectric layer. Thus, any improved means of heating pattern control should be compatible with the typical constructions of interstitial probes, whether they be of the flexible or rigid type.